1. Field of the Invention
The present invention relates to a vehicular mountable fuel cell system suitable for vehicular mounting, comprising a combustor for combusting off gas from a fuel cell supplied with reformed gas from a reformer, and a turbine compressor for supplying air to the fuel cell by compressing air at a compressor unit by utilizing combustion energy of the combustor for recovering power by effectively utilizing waste gas.
2. Description of the Related Art
It is known in a polymer electrolyte type fuel cell (PEFC) that when the gas pressure of hydrogen and oxygen used in the reaction is increased, the electrode reaction is activated and an output level of the cell is elevated. However, for an increase in the supply gas pressure, the power of the air compressor used as a supply source of oxidizing air must also be increased and so the net output of the gas pressurizing system is not necessarily improved.
Further, 100% of the hydrogen and oxygen supplied to a fuel cell is not consumed but 20 through 50% of the hydrogen and 30 through 70% of the oxygen (in respect of supply amounts) are exhausted as waste gas. As methods of utilizing the waste gas, there have been proposed a method for resupplying the waste gas to a fuel cell stack FC by installing recirculating lines RL as shown by FIG. 9 (JP-A-8-203547), and a method of utilizing the waste gas as a fuel for a burner for an evaporator V of a reformer K as shown by FIG. 10 (JP-A-5-283091).
Further, although a fuel cell (FC) constitutes a noiseless and nonpollutant energy source, a key point for high efficiency of the overall system resides in how to reduce the power of an air supply apparatus such as an air compressor. In an FC power generating apparatus which is referred to as a normal pressure type which is operated at a pressure near to atmospheric pressure, the required power of the air supply apparatus is small but the total power generated is small and so the overall efficiency is not necessarily regarded as excellent.
Similarly, in a pressurized type system, the power generated by the FC is remarkably increased but the power required for the air supply apparatus is also increased. An actual system is operated at a pressure producing the best balance of the system efficiency in line with the actual components, or the system efficiency is increased by controlling the pressure of the air supply apparatus in accordance with a required load.
Further, according to a fuel cell power generating apparatus of a solid polymer membrane type, the ionic conductivity of the membrane-needs to increased by humidifying the solid polymer membrane. For that purpose, a filel gas (H.sub.2 or the like) and an oxidant gas (air or the like) are humidified to thereby supply moisture to the solid polymer membrane. As a humidifying method, there is generally carried out bubbling or directly supplying water W to a fuel gas F and an oxidant gas O as shown by FIG. 1 (JP-A-7-326376). According to the conventional method of resupplying waste gas to the fuel. cell stack FC by installing the recirculating lines RL, when reformed gas is used, there occurs a problem that when the reformed gas is recirculated, the concentration of hydrogen is reduced.
Further, according to the method of utilizing waste gas as fuel for a burner for the evaporator V of the reformer K, although this method is realistic, there occurs a problem in which the effectiveness differs depending on the constitution of the system.
In the normal pressure type, when characteristics of FC or the power of the auxiliary machines are changed, there occurs a problem that a balance point needs to be reevaluated and, depending on operating conditions, a very inefficient system may result. Further, in the case of the pressurizing type, much time is taken in controlling the pressure of the air supply apparatus and the method is unsuitable for a system in which the load is rapidly changed as occurs particularly in a FC for a vehicle.
In the bubbling operation mentioned above, there is needed a heat source for a bubbler. Further, according to the method of supplying water W directly to the gases F and O, there occurs a problem in that the temperature of the gases may be lowered by the humidifying operation, or water clogging may be caused in a gas pipe or in the fuel cell.
As a conventional means for resolving such a problem, there has been adopted a method of heating the humidifying water and elevating the temperature thereof by thermally exchanging heat of an off gas from a fuel cell and the reaction heat of the fuel cell with the humidifying water. However, the temperature stays low due to a deficiency in the heat amount when such heat is used, and accordingly sufficient heat exchange is difficult.